The overall objective of the proposed research is to determine how cyclic nucleotides function in visual photoreception. We are studying the following problems: 1) How does bleaching of rhodopsin control the activity of adenyl cyclase in retinal rod outer segments? I postulate that cyclase activity is directly controlled by disc membrane potential, and that the latter varies with rhodopsin conformation (e.g., the early receptor potential) and with Ca ion release. I propose to control and measure disc membrane potential using techniques we and others have developed in the study of photosynthetic phosphorylation. These include the use of ionophoretic antibiotics and intrinsic and extrinsic membrane probes. The effect of membrane potential on cyclase activity can then be measured. 2) How do rod outer segment microtubules interact with cyclic AMP? Do isolated ROS microtubules contain guanine nucleotides, and do they have enzymatic activity in addition to cyclic AMP-stimulated protein kinase? 3) By what mechanism do phosphodiesterase inhibitors reversibly eliminate the light response of the crayfish caudal photoreceptor unit? The effect of caffeine, aminophylline and papaverine on processes such as light and dark adaptation will be determined. Since aminophylline inhibition of the frog late receptor potential can be reversed by the establishment of an artificial Na ion gradient, this experiment will also be performed with the crayfish photoreceptor.